First, reinstall the battery and connect your power adapter. Hold down the <Command (Apple)> and S keys immediately after pressing the power button until you see the rotating wait icon. The screen should go black, then display text describing startup operations. When this is done, at the command prompt (your username), type "fsck -f" (leave off the quotes), then <return>. This will run the Unix disk repair utility. If it finds errors, when it is done fixing them, press the up-arrow key to bring back the fsck -f command, and press <return> once more to verify all errors were fixed - sometimes it takes multiple passes. Once all of the errors are repaired, type "reboot" and let the computer restart normally. If this works, you're good to go. If not, continue with the following paragraph.
If you don't have your system disks, try removing the hard drive (diagnostic step). First, remove the battery. At the end of the battery compartment closer to the center of the laptop there is a chrome-plated metal strip held in place with some small (#0 Phillips) screws. Remove these screws and the strip. There will be a tab attached to the hard drive. Pull it and drag the hard drive into the battery compartment, then (gently! It's fragile) set it aside. Now try turning on your computer. It should display a flashing question mark. If it does, the computer is working normally, but your operating system is corrupted or the hard drive is bad (most likely the former problem). In this case, put the hard drive back in, then go to the Apple web store and order a copy of Mac OS 10.6
Mac OS 10 6 Snow Leopard . You'll need it whether the hard drive is bad or good. Use Disk Utility to scan the drive - it may be able to fix the problem on its own. If not, then try a System Install. If Disk Utility on the CD can't find the hard drive or reports it is not repairable, try formatting the drive (everything on it will be erased, but it's better than nothing at all). If that won't fly, you'll have to buy a new hard drive (don't recycle the old one until you get the new one; you'll need the mounting hardware from the old drive). You can use any SATA II and almost any SATA III type laptop hard drive in your computer. If you are willing to sacrifice some battery life for greater performance, get a 7200 RPM drive instead of a 5400 RPM unit, or if you have lots of money and want both battery life and great performance, get a flash drive.
I recommend also getting an external drive to back up your system with Time Machine ASAP. The 80 GB hard drive used in the MacBook has a reputation for head breakage. When this happens, the disk is so thoroughly trashed that neither professional recovery services nor even the NSA can get anything out of it.